[Evolution] Evolution won't keeps collapsing some threads

[Brian Mury <b mury ieee org>]

All of my Evolution folders use a threaded view with the threads all
expanded.

Recently one of my folders starting collapsing some (not all) of the
threads. I can expand them all, but as soon as I do anything that causes
Evolution to redo the threading (such as delete a message, or turn
threading off and back on), it collapses some of the messages again.

How do I tell it that I want the messages to stay expanded? It's never
done this before!